1. Field of the Invention
This invention relates broadly to radomes. More particularly, this invention relates to retractable forward looking radomes. The invention has particular applicability to radomes for supersonic aircraft, although it is not limited thereto.
2. State of the Art
Aircraft utilize radar to assist in navigating when visibility is decreased due to atmospheric conditions. Weather radar devices, operating within X-band at approximately 9.345 GHz, permit pilots to locate and navigate through or around stormy weather. Weather radar can locate and indicate storm conditions, but cannot provide television-type images. A synthetic vision, millimeter wave (mm-wave) imaging radar system is currently being developed which operates within W-band at 94 GHz. It has been found that at 94 GHz there is an atmospheric window which permits radar to image through fog. A narrow beam width of the 94 GHz radar is transmitted from the radar system of the aircraft through the fog. The pilot of the aircraft utilizes a heads up display (HUD) to visualize the image obtained from the 94 GHz radar. The HUD includes a pull-down transparent glass screen, similar to a sun visor, and a projector above the pilot which projects an image of the airfield onto the glass screen. The image of the HUD is boresighted (aligned) with the pilot's view of the airfield. This imaging radar is important during landing and take-off in poor visibility weather conditions and during the night, but is not required during other phases of flight.
A radome is an electromagnetic cover for the radar system of an aircraft. Different types of radomes are known. Military aircraft are often provided with retractable radomes for use with surveillance systems. For example, U.S. Pat. No. 3,754,267 to Walters et al., U.S. Pat. No. 3,766,561 to Johnson, U.S. Pat. No. 3,982,250 to Giannatto et al., and U.S. Pat. No. 4,593,288 to Fitzpatrick each disclose retractable radomes. All of the known retractable radomes are provided for military or police surveillance and are designed to scan in 360 degrees in azimuth. On commercial air transport aircraft, i.e., passenger planes, the nose of the aircraft is a radome. The radomes on a commercial air transport aircraft permit scanning only in a forward direction, and do not provide 360 degree scanning.
When a radar system is mounted onto an aircraft it is necessary to cover the system with a radome which will protect the radar system from the environment, shielding the system from heat, wind, and rain. It is also desirable for the radome to provide a light-weight housing for the system which conforms to the contours of an aircraft (if not retractable) and provides for a low aerodynamic drag shape. In satisfying these requirements, it is important that the radome not substantially adversely affect the radar when the radar energy passes through the radome and also when the reflected radar energy enters back through the radome to be received by the radar antenna.
The transmission efficiency of a radome is measured by a radome's ability to minimize reflection, distortion and attenuation of radar waves passing through the radome in one direction. The transmission efficiency is analogous to the radome's apparent transparency to the radar waves. As radomes are electromagnetic devices, transmission efficiency can be optimized by tuning the radome. The tuning of a radome is managed according to several factors, each of which is a function of the transmission frequencies of the aircraft's radar, including wall thickness, dielectric constant, and loss tangent of the materials.
A radome should have a relatively constant transmission efficiency over the scanning range of the radar system, behaving substantially the same when transmitting radar at various beam to wall angles. For example, when the radar system is transmitting and receiving out of the side of the nose of the plane, the reflection, attenuation, and distortion should not be unacceptably different than when the radar is transmitting out of the front of the nose of the plane.
Supersonic aircraft are provided with a highly pointed nose radome which may house weather radar and instrument landing system (ILS) antennas. The highly pointed nose radome has very shallow beam to radome wall incidence angles when the radar energy is transmitting out the front of the radome. The shallow beam to wall incidence angles inhibit effective transmission of mm-wave radar energy and result in strong reflection lobes. This problem is especially true for the nose radome of the High Speed Civil Transport (HSCT) Mach 2.5 supersonic airliner currently being developed by NASA and United States aircraft companies. The proposed nose design for this aircraft is highly pointed, even more so than the Concorde. With incidence angles approximately between 70.degree. and 80.degree. , the nose design is a radio-frequency unfriendly shape. As a result, it would be very difficult, if not impossible, to design a nose radome for this supersonic aircraft, or for any other supersonic aircraft, which would effectively house a mm-wave radar system in the same confines as the other radar systems and efficiently transmit mm-wave radar energy. In addition, X-band weather radar energy transmission is also inhibited by shallow beam to wall incidence angles. Furthermore, a new predictive wind shear detection mode available for X-band weather radar requires the use of a radome which has low side lobe and reflection lobe levels. The level of performance required for the predictive wind shear detection mode may not be possible with present radome technology for a radome designed to be the proposed shape of the nose of the HSCT.
In addition, large business jets, e.g., the GulfstreamIII, generally do not have sufficient space to house the ILS antenna and weather and imaging radar antennas, such that each of the antennas is capable of being oriented to transmit energy through the radome in a forward direction at beam to wall incidence angles which will produce adequate transmission efficiency. Also smaller business jets, e.g., the Learjet and the Cessna Citation, and commuter airliners, e.g, the DeHavilland DHC-8 and the Saab 340, do not have sufficient space available to accommodate an imaging radar antenna in addition to the existing weather radar antenna.